Cell mediated cytotoxicity has been shown to be a major component of the immune responses directed against allogeneic tissues. Leu-Leu-OMe treatment of human or murine leukocytes has been observed to deplete natural killer cells, cytotoxic T lymphocytes (CTL), CTL precursors and cells of myeloid origin, while T helper cells, B cells and cells of non-bone marrow origin remain functionally intact. Moreover, Leu- Leu-OMe treatment of donor cells prevents development of lethal graft-versus-host disease in a semi-allogeneic MHC disparate murine model of bone marrow transplantation. The selective killing of cytotoxic lymphocytes and myeloid cells by this agent has been shown to be dependent upon the, presence of high levels of dipeptidyl peptidase I (DPPI) within the specialized granules of these cells. The overall goals of this project are to make use of new information on the biochemistry of Leu-Leu-OMe and DPPI to probe a number of aspects of the physiology of cytotoxic cells, the biology of their granules and the role of these cells in graft versus host disease and allograft rejection. The specific aims are: (1) Assess the role of DPPI generated membranolytic (Leu-Leu)n-OMe products in inducing apoptosis within DPPI- enriched cells; (2) Test the proposition the DPPI plays an obligate role in post-translational processing of murine granzymes A and B; (3) test the hypothesis that inhibition of DPPI activity during CTL activation impairs effector functions dependent upon granule serine protease activity; and (4) delineate the role of DPPI enriched cells and the specific role of DPPI enzymatic activity in the evolution of in vivo alloimmune responses.